Why is your foot

When we kick a football due to its lightweight it moves and gives less recoil to us due to the law of conservation momentum and when we kick a heavy stone it gives us more recoil to conserve momentum and has more inertia of rest, so we get hurt by kicking a stone

OR

Grinding stone is like a wheel and at every point of the wheel at the periphery, the direction of linear velocity is tangential due to the inertia of motion, so that is why the sparks go tangential